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Thursday, May 23, 2013

Caves of the YucatánPhysical Geology 2004

The Yucatán Peninsula is composed of a large and intricate system of beautiful underwater caves and cenotes. These caves have formed formed from a combination of varying geologic phenomenon such as glaciation, dissolution and the impact of a large asteroid.

For millions of years, the Yucatán Peninsula of Mexico was submerged beneath a prehistoric ocean and was largely made up of coral reefs. These reefs thrived in the shallow and warm waters of their environment and lithified to form over 1300 m of limestone strata during the Cretaceous period. During the Tertiary period, another 1000 m of carbonate deposits accumulated, creating the great limestone platform that makes up the Yucatán Peninsula today. Due to the carbon dioxide present in the atmosphere, precipitation in the area is slightly acidic. This acidic water is thus able to dissolve and percolate through the porous limestone until it reaches the aquifer below the surface. The combination of the aquifer and acidic precipitation carves the long caverns that characterize the peninsula today.

Throughout the Sangamon Interglacial and the Wisconsin post-glacial periods, the sea level fluctuated. As the sea level dropped during active glaciation, the water table also dropped, leading to the drainage of caverns that were previously filled with water. This caused the ceilings of the caves to collapse because they were no longer supported by the water, thus creating new karst windows that carried stream beds along their bottoms. In time, this karst water eventually eroded the limestone walls of the caves leading to the broadening and lengthening of cave passages. As the sea level rose again, these passages, initially filled with fresh water, mixed with the incoming salt water from the ocean.

Cenotes and the Maya

Another geologic formation that characterizes the Yucatán Peninsula is the cenote (pronounced say-no-tay). This word is derived from the Mayan word, 'Dzonot' which means; sacred well. More specifically, the word cenote refers to a large natural sink hole whose limestone covering has caved in years ago to reveal the running stream below. While some of these structures are simply vertical shafts filled with water, others are composed of underwater passageways.

Cenotes have played a significant role in the lives of the Mayan Indians of the Yucatan area and continue to do so today. Because this particular region of Mexico is fairly dry, with relatively no rivers or streams on the land surface, the underground water beds provide an important source of water for the mayan populations. The Mayas were able to utilize the opening of the cenotes to retrieve water much like Westerners use a well. Because these underground water systems were very extensive and deep, it is not surprising that the Mayan civilizations built around cenotes were able to thrive.

Along with being used for subsistance purposes, the cenotes and caves of the Yucatan held significant spiritual meaning in Mayan culture. They were thought of as entrances to the underworld, called Xibalba, where the Mayan gods and ancestors could be contacted by the living. Many sacred rituals and ceremonies were therefore held within these underground chambers because they were considered to be closer to divine and supernatural powers. It is for this same reason that many archaeologists speculate that cenotes were used as a site for human sacrifice and burial. Though numerous skeletal remains of ancient Mayans have been found within these structures, the notion of their use for such purposes is questionable. For one thing, the decomposition of bodies within centotes would have certainly contaminated the Mayan's only water source. In addition, many of the Mayan civilizations built around cenotes had been inhabited for hundreds of years, meaning that far more bodies would have been discovered had sacrifice and burial been a common practice within cenotes.

Effects of the Chicxulub Impact Crater

During the nineteenth century, as geologists were studying layers of geologic strata, a distinct difference in fossil assemblage was recognized between rocks of the Cretaceous and Tertiary periods (about 65 million years ago). Through modern methods of dating, geologists were later able to confirm that this worldwide change in fossils occured instantaneously and represents the extinction of most of the species living on Earth, most notably the dinosaurs.

In order to explain this worldwide extinction and the close of the Cretaceous period, most geologists have concluded that a 10-km wide bolide collided with the Earth's surface near the present day site of the Yucatan Peninsula (see map above). They believe that the impact of this extraterrestrial object blasted large amounts of debris into the atmosphere, which blocked a substantial amount of sunlight and led to a dramatic drop in the Earth's climate. The impact is also believed to have created 2-km high tsunamis and generated enough heat to set forests on fire. The Chicxulub crater is the result of this bolide impact.

There is an important link between the formation of this crater and the cenotes and caves of the Yucatan described above. When the bolide collided with the surface of the Earth, it created a ring fault bounding the impact crater. This boundary fault intercepts the flow of groundwater, diverting it up and around the fault line so that the water dissolves the overlying strata and has thus created the caves and cenotes of the Peninsula described above.